The present invention relates to integrated circuits of the MOS variety and more particularly to the voltage and/or signal supply for the active elements in such a circuit, called MOSFETs, for operation and bias thereof.
It is a known fact that integrated circuit chips as made in large quantities vary to some extent from batch to batch. These variations may stay within specified tolerances, and they may not, and whenever these tolerances are exceeded, the chip (or even an entire batch) is not deemed useable. Process control of course will tend to keep to the yield high, but, it has to be observed that the tolerances deemed still acceptable are, to some extent, arbitrary and the result of compromise. Principle criterion is adequate operation of a chip as such and within particular performance requirements particularly with regard to adequate cooperation with other chips (usually not from the same batch) as well as with other e.g. discrete circuit elements.
As integrated chips cooperate with others, the signal and other voltage levels applied and passing between them must be and are usually based on averages which are deemed adequate for operation of all circuit elements and chips of the interconnected system, and the tolerances have been referenced with regard to that average.
Integrated circuit chips are usually used as and in logic circits so that the essential internal criterion is the ready distinction between logical or digital "ones" and "zeros". This distinction is usually defined and established by distinguishing between conduction and nonconduction of FET elements in the chip. Logic states or numbers "one" and "zero" propagate through the chip from element to element until combined with others to form a new output state. If that propagation is extensive, it cannot be expected that the voltage levels defining that state remain invariable. Moreover, control voltages are usually established internally as node charges and they must be expected to differ in level for similar logic states they are supposed to define. Maintaining distinguishable voltage levels here is the primary objective, and for attaining that objective, one has to define tolerances as criterion (usually as test criterion to separate the useful devices from those that might cause difficulties), under which conditions a FET should or should not conduct. The yield of MOSFET chip processing is directly related to the closeness of these performance tolerances.